High aspect ratio contact structure with reduced silicon consumption

ABSTRACT

A high aspect ratio contact structure formed over a junction region in a silicon substrate comprises a titanium interspersed with titanium silicide layer that is deposited in the contact opening and directly contacts an upper surface of the substrate. Silicon-doping of CVD titanium, from the addition of SiH 4  during deposition, reduces consumption of substrate silicon during the subsequent silicidation reaction in which the titanium reacts with silicon to form a titanium silicide layer that provides low resistance electrical contacts between the junction region and the silicon substrate. The contact structure further comprises a titanium nitride contact fill that is deposited in the contact opening and fills substantially the entire contact opening.

RELATED APPLICATION

This application is a continuation of prior application Ser. No.10/931,854, filed Sep. 1, 2004, now U.S. Pat. No. 6,908,849 which is adivisional application of application Ser. No. 09/944,903, filed Aug.30, 2001, now U.S. Pat. No. 6,858,904, which are hereby incorporated byreference in their entirety. This application is also related toco-pending application Ser. No. 09/945,065, filed Aug. 30, 2001, whichis hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to integrated circuits, and more particularly, toa high aspect ratio contact structure with reduced consumption ofsilicon in the junction region.

2. Description of the Related Art

A high density integrated circuit typically includes numerous electricaldevices and conductors formed on multiple layers of conducting andsemiconducting material that are deposited and patterned in sequenceonto a substrate surface. An integrated circuit is operable when itsindividual components are interconnected with an external source andwith one another. In particular, designs of more complex circuits ofteninvolve electrical interconnections between components on differentlayers of circuit as well as between devices formed on the same layer.Such electrical interconnections between components are typicallyestablished through electrical contacts formed on the individualcomponents. The contacts provide exposed conductive surfaces on eachdevice where electrical connections can be made. For example, electricalcontacts are usually made among circuit nodes such as isolated deviceactive regions formed within a single-crystal silicon substrate.However, as the contact dimensions of devices become smaller, thecontact resistance and the sheet resistance of the contacts alsoincrease.

To address this problem, refractory metal silicides have been used forlocal interconnections to provide low resistance electrical contactsbetween device active regions within the silicon substrate. One commonmethod of forming metal silicides is a self-aligned silicide process,often referred to as silicidation. In this process, a thin layer ofrefractory metal, such as titanium, is deposited over a dielectric areaand through contact openings formed on the dielectric area to contactunderlying silicon circuit elements, such as a source and drain activeregions formed within a silicon substrate. The structure is thenannealed to form a silicide, such as titanium silicide (TiSi_(x)), at ahigh temperature. During annealing, the deposited titanium reacts withthe silicon in the substrate to form TiSi_(x) inside the contactopenings adjacent the active regions. The titanium and silicon reactwith each other to form a silicide thick enough to provide low sheetresistance. The process is referred to as “self-aligning” because theTiSi_(x) is formed only where the metal layer contacts silicon, forexample, through the contact openings. As such, titanium that overliesthe dielectric areas surrounding the contact openings, along thesidewalls of the openings, and any other non-silicon surfaces remainsunreacted.

The conventional silicidation process is not entirely suitable fordevices having relatively shallow contact junctions. Shallow junctionstructures may be damaged when the silicidation reaction consumes adisproportionate amount of silicon from the relatively shallow junctionregion. To address this problem, a titanium silicide film can bedirectly deposited on the silicon substrate to reduce siliconconsumption in the junction area. The TiSi_(x) film can be depositedusing low pressure chemical vapor deposition (LPCVD) or chemical vapordeposition (CVD) processes. However, there are numerous disadvantagesassociated with forming a conventional TiSi_(x) film on the substrate.For example, the LPCVD process used typically requires reactiontemperatures in excess of 700 C and the conventional CVD process tendsto produce a TiSi_(x) film with high bulk resistivity.

Moreover, subsequent to forming the TiSi_(x) film, a diffusion barrierlayer such as titanium nitride (TiN) is typically formed on the contactstructure. The TiN layer inhibits subsequently deposited contact metalfrom diffusing into the insulating layer surrounding the contactstructure. Typically, TiN is deposited on the TiSi_(x) layer in thecontact openings as well as on the unreacted Ti remaining on thedielectric layer and on the sidewalls of the contact openings.Disadvantageously, TiN forms a relatively weak bond with Ti and islikely to peel off from surfaces where TiN has contact with Ti. Toaddress this problem, the Ti deposited on the dielectric layer and onthe sidewalls of the contact opening can be removed prior to depositionof TiN. However, the Ti removal process is likely to add to the cost andcomplexity of the fabrication process.

Furthermore, once the diffusion barrier layer is formed, conductivecontact fills such as tungsten can be deposited into the contactopenings. The contact fills are typically deposited into the contactopenings by physical deposition processes such as sputtering. However,the step coverage provided by sputtering and other physical depositionprocesses is often inadequate for high aspect ratio contact openingsbecause it can be particularly difficult to physically deposit uniformlayers of contact fill into high aspect ratio contact openings.

Hence, from the foregoing, it will be appreciated that there is a needfor a method of improving the step coverage of contact fills in highaspect ratio contact structures. There is also a need for a contactstructure having improved contact fill adhesion. Furthermore, there isalso a need for a method of reducing silicon consumption in shallowjunction regions during silicidation process. To this end, there is aparticular need for a high aspect ratio contact structure that providesa more uniform step coverage and improved TiN adhesion. There is also aparticular need for a method of reducing silicon consumption in shallowjunction regions during the formation of the titanium silicide layer.

SUMMARY OF THE INVENTION

In one aspect, the preferred embodiments of the present inventioncomprise an integrated circuit having a silicon substrate and aninsulating layer formed on an upper surface of the substrate wherein acontact opening is formed in the insulating layer. Preferably, thecontact opening extends from an upper surface of the insulating layer tothe upper surface of the substrate. Furthermore, a conductive contact isdeposited in the opening in a manner such that the conductive contactdirectly contacts the upper surface of the substrate. Preferably, theconductive contact comprises a titanium layer interspersed with titaniumsilicide. The integrated circuit further comprises a conductive contactfill that is deposited on an upper surface of the conductive contact ina manner such that the contact fill fills substantially the entirecontact opening. Preferably, the contact fill comprises titanium nitridethat is chemically deposited into the opening and conforms to thecontours of the opening.

In another aspect, the preferred embodiments of the present inventioncomprise a contact structure having a contact opening formed over ajunction region in a silicon substrate. The contact structure comprisesa conductive contact layer comprising titanium interspersed withtitanium silicide. In one embodiment, the layer of titanium interspersedwith titanium silicide is approximately 100 Å thick. In anotherembodiment, the titanium layer interspersed with titanium silicidecomprises approximately 10% silicon. Preferably, the conductive contactlayer is deposited directly on an upper surface of the silicon substrateover the junction region. In one embodiment, the junction region is lessthan about 1 μm deep. Advantageously, the titanium silicide in theconductive contact layer reduces consumption of silicon from thejunction region during a subsequent silicidation reaction betweensilicon in the substrate and titanium in the conductive contact layer.The contact structure further comprises a diffusion barrier layer thatis formed on an upper surface of the conductive contact layer. A contactfill is also formed on an upper surface of the diffusion barrier layer.Preferably, the contact fill comprises titanium nitride thatsubstantially fills the entire contact opening.

In yet another aspect, the preferred embodiments of the presentinvention comprise a method of fabricating a contact structure on asilicon substrate. The method comprises forming an insulating layer onan upper surface of the substrate and forming an opening in theinsulating layer. Preferably, the opening extends from an upper surfaceof the insulating layer to the upper surface of the substrate. Themethod further comprises forming a titanium layer interspersed withtitanium silicide in the opening. Preferably, the titanium layerinterspersed with titanium silicide is deposited in opening and directlycontacts the upper surface of the substrate. The titanium layersubsequently reacts with silicon in the substrate to form a titaniumsilicide layer. Advantageously, the titanium silicide interspersed inthe titanium layer reduces the consumption of silicon during theformation of the titanium silicide layer. The method further comprisesforming a conductive contact fill in the opening. Preferably, theconductive contact fill comprises titanium nitride that is depositeddirectly on the titanium silicide layer and fills substantially theentire opening. These and other advantages of the present invention willbecome more fully apparent from the following description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross sectional view of a partiallyfabricated integrated circuit of one preferred embodiment of the presentinvention;

FIG. 2 illustrates a schematic cross sectional view of the integratedcircuit of FIG. 1, showing the formation of a metal layer in the contactopening;

FIG. 3 illustrates a schematic cross sectional view of the integratedcircuit of FIG. 2, showing the formation of a metal silicide layeradjacent the substrate;

FIG. 4 illustrates a schematic cross sectional view of the integratedcircuit of FIG. 3, showing the formation of a metal silicidationadhesion layer;

FIG. 5 illustrates a schematic cross sectional view of the integratedcircuit of FIG. 4, showing the formation of a metal nitride diffusionlayer;

FIG. 6 illustrates a schematic cross sectional view of the integratedcircuit of FIG. 5, showing the formation of a contact fill in thecontact opening.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

References will now be made to the drawings wherein like numerals referto like parts throughout. While the preferred embodiments areillustrated in the context of contact openings over active regions insilicon substrates, it will be recognized by one skilled in the art ofsemiconductor fabrication that the invention will have applicationwhenever electrical contact to silicon elements is desirable.Furthermore, the term “substrate” as used in the present application,refers to one or more semiconductor layers or structures which includeactive or operable portions of semiconductor devices.

FIG. 1 illustrates a schematic sectional view of a semiconductorstructure 100 of the preferred embodiment. As FIG. 1 shows, thesemiconductor structure 100 generally comprises a substrate 102 having ahighly doped silicon active area 104, which may comprise a transistorsource or drain, defined below an upper surface 106 of the substrate102. Furthermore, two gate structures 108 a, 108 b are formed over thesilicon substrate 102 adjacent the active area 104. Each of the gatestructures 108 a, 108 b has a thin gate oxide layer 110 a, 110 b, apolysilicon gate electrode layer 112 a, 112 b, a metallic layer 114 a,114 b, a protective cap layer 116 a, 116 b, and side wall spacers 118 a,118 b, 120 a, 120 b to protect the gate structures. As FIG. 1 furthershows, an insulating layer 122 is formed over the gate structures 108 a,108 b and the active area 104 on the silicon substrate 102. Preferably,the insulating layer 122 is comprised of borophosphosilicate (BPSG) orother generally known insulating material.

With reference to FIG. 2, a contact opening 124 is formed through theinsulating layer 122 over the active area 104 to provide electricalcontact to the active area 104. Preferably, the contact opening 124 isdefined by the insulating layer 122, inner side wall spacers 118 b, 120b, and the active area 104. In one embodiment, the contact opening 124has an aspect ratio of at least 10:1. In another embodiment, the contactopening 124 has an aspect ratio of at least 5:1. As FIG. 2 furthershows, a layer of metal 126 is subsequently formed in the contactopening 124 and over the insulating layer 122. In one embodiment, themetal layer 126 comprises titanium (Ti). As shown in FIG. 2, most of theTi is formed over an upper surface 106 of the active area 104 orjunction region and the insulating layer 122 surrounding the contactopening 124 although some remaining titanium may also form on the sidewalls 123 of the contact opening 124 and top surface of the dielectricduring the deposition process. Preferably, Ti is deposited using aplasma enhanced chemical vapor deposition (PECVD) process. In oneembodiment, the PECVD process uses a gas mixture comprised of TiCl₄, Ar,H₂, and He. Furthermore, the reaction gas temperature is preferablyabout 650° C., the RF power is approximately 400 W and the chamberpressure is about 4 Torr.

The metal layer 126 is subsequently annealed during which the metalformed on the substrate surface 106 above the active area 104 reactswith silicon in the substrate 104 to form a layer of metal silicide 128as shown in FIG. 3. As previously discussed, the metal silicide layer128 is formed to provide low resistance electrical contacts betweendevice active regions within the silicon substrate, particularly in highaspect ratio contact areas where the contact resistance is relativelyhigh. In one embodiment, the metal silicide layer 128 comprises titaniumsilicide (TiSi_(x)). As shown in FIG. 3, the TiSi_(x) layer 128 isformed over areas where Ti has contact with silicon in the substratewhile portions of the Ti layer 126 deposited on the top of theinsulating layer 122 and sidewalls 123 of the contact opening 124remains unreacted.

In an alternative embodiment, during deposition of the metal layer 126,the metal can be doped with a small amount of silicon to form the metalsilicide layer 128 on the substrate surface 106. In a preferredembodiment, titanium doped with silicon is deposited on the substratesurface by a PECVD process using a mixture comprising TiCl₄, Ar, H₂, He,and SiH₄. In one embodiment, the process temperature is 650° C., RFpower 400 W and chamber pressure 4 Torr. Preferably, a small amount ofabout 10 sccm of SiH₄ is added to the gas mixture at about 400 W.Preferably, the process deposits a titanium rich layer interspersed withTiSi_(x) formed by reactions between the deposited silicon and some ofthe titanium.

This embodiment is particularly useful for forming contact structuresover shallow junction regions where titanium may consume sufficientsilicon from the junction region to adversely affect the electricalintegrity of the contact. In particular, leakage in the junction canoccur when a disproportionate amount of silicon is consumed by thetitanium. Advantageously, doping titanium with a small amount of siliconreduces consumption of silicon from the junction region and produces atitanium rich TiSi_(x) film having improved chemical and mechanicalproperties. Furthermore, the silicon doped titanium layer does notappear to affect the electrical integrity of the contact.

With reference to FIG. 4, subsequent to forming the metal silicide layer128 adjacent the active area 104, a metal silicide adhesion layer 132 isformed on an upper surface 134 of the titanium film 126 deposited on theinsulating layer 122 and side walls 123 of the contact opening 124. Themetal silicide adhesion layer 132 preferably comprises TiSi_(x) that isdeposited by a PECVD process using a gas mixture comprising SiH₄, TiCl₄,Ar, H₂, and He. In one embodiment, the TiSi_(x) adhesion layer 132 isdeposited at a temperature of about 650° C., RF power of about 400 W andchamber pressure of about 4 Torr. Preferably, approximately 10 sccm ofSiH₄ is introduced to the reaction process at about 400 W as a source ofSi. In one embodiment, the TiSi_(x) adhesion layer 132 is formed on thepreviously deposited Ti film 126 to promote adhesion between the Ti film126 and a subsequently deposited contact fill. According to one theory,the Ti layer 126 contains an appreciable amount of chlorine that is leftover from the PECVD reaction gas. It is believed that the chlorinepresent in the Ti layer tends to inhibit formation of stable chemicaland mechanical bonds with TiN. The TiSi_(x) adhesion layer on the otherhand contains far less chlorine than the Ti layer and has chemical andmechanical properties that are more conducive to forming strong andstable bonds with TiN. In one embodiment, the TiSi_(x) adhesion layercan be formed immediately following the Ti deposition process using thesame equipment and substantially the same process parameters.

As illustrated in FIG. 5, following the formation of the TiSi_(x)adhesion layer 132, a metal nitride diffusion barrier layer 136 isformed on the TiSi_(x) adhesion layer 132. Preferably, the metal nitridelayer 136 comprises TiN that is deposited by a thermal CVD process fromTiCl₄ and NH₃ precursors. In one embodiment, the processing temperatureis approximately 600 C. The metal nitride layer 134 is typically used asa barrier layer against junction spiking and diffusion of metal into theinsulating layers. As such, it is desirable for the metal nitride layerto form a stable and durable bond with the contact structure.

However, as previously discussed, metal nitrides layers such as TiNgenerally do not adhere well to the Ti metal 126 deposited on thesidewalls 123 of the contact structure and top surface of thedielectric. Consequently, the weak and unstable bonding between TiN andTi often leads to TiN peeling off from the sidewalls 123 of the contactstructure and top surface of the dielectric, particularly at locationswhere TiN makes contact with Ti. Advantageously, the contact structure100 of the preferred embodiment interposes the TiSi_(x) adhesion layer132 between the Ti 126 and TiN 136 layers whereby the TiSi_(x) serves asa “glue” that bonds together the Ti and TiN layers. As such, theadhesion between TiN and Ti layers can be substantially improved and theoccurrence of TiN peeling is substantially reduced. Furthermore, theformation of the TiSi_(x) adhesion layer also eliminates the separateprocess that would otherwise be required to remove the remaining Ti film126 from the sidewalls 123 of the contact opening 124 and top surface ofthe dielectric. Thus, the contact structure of the preferred embodimentprovides a high aspect ratio opening with uniform metal coverage andsuperior adhesion of diffusion barrier layer and can be manufacturedefficiently and cost-effectively.

Subsequent to forming the TiN diffusion barrier layer, a contact fill138 is deposited in the contact opening 124 as shown in FIG. 6. In oneembodiment, the contact fill 138 can comprise a metal such as tungstenor copper and can be deposited using a physical deposition process suchas sputtering. In another embodiment, the contact fill 138 is comprisedof TiN that can be deposited using a PECVD process from precursors suchas TiCl₄ and TiI₄. Advantageously, contact fills comprised of TiN areparticularly suited for high aspect ratio contact openings as the TiNfill can be deposited using, for example, chemical vapor depositiontechniques which provide superior step coverage. Furthermore, contactfills comprised of TiCl₄ TiN also have superior electrical conductivitywhen compared with most other conventional contact fill materials.

As described above, the contact structures of the preferred embodimentsutilize a metal silicide adhesion layer to improve the adhesion betweenthe metal and metal nitride layer in a contact structure. The metalsilicide adhesion film can be fabricated cost effectively using existingequipment and processes. The preferred contact structures also utilize achemically deposited metal nitride as contact fill so as to improve thestep coverage of the contact fill. The improved step coverage isparticularly desirable for high aspect ratio contact openings in whichsuperior step coverage is difficult to achieve when using conventionalmetal contact fills. Moreover, the contact structures of the preferredembodiments also comprise a titanium rich titanium silicide layer thatis formed without consuming a significant amount of silicon from thecontact junction regions, which is particularly desirable for shallowjunction regions that can be easily damaged during the conventionalsilicidation process.

Although the foregoing description of the preferred embodiment of thepresent invention has shown, described and pointed out the fundamentalnovel features of the invention, it will be understood that variousomissions, substitutions, and changes in the form of the detail of theapparatus as illustrated as well as the uses thereof, may be made bythose skilled in the art, without departing from the spirit of theinvention. Consequently, the scope of the invention should not belimited to the foregoing discussions, but should be defined by theappended claims.

1. A method of fabricating a contact structure on a silicon substrate,comprising: forming an insulating layer on an upper surface of thesubstrate; forming an opening in the insulating layer, wherein theopening extends from an upper surface of the insulating layer to theupper surface of the substrate; and depositing a metal rich layerinterspersed with a metal silicide, wherein the metal silicide is formedby reacting said metal with a dopant; subsequently reacting the metalrich layer interspersed with a metal silicide with said silicon in thesubstrate, wherein the metal silicide interspersed in the metal richlayer reduces consumption of the silicon in the substrate.
 2. The methodof claim 1, wherein the dopant is silicon.
 3. The method of claim 1,wherein said metal rich layer interspersed with a metal silicidecomprises a titanium rich layer interspersed with titanium silicide. 4.A method of reducing consumption of silicon in a silicon substrate in asilicidation reaction: depositing a metal rich layer interspersed with ametal silicide on an upper surface of said silicon substrate, whereinthe metal silicide is formed by reacting said metal with a dopant; andsubsequently reacting said metal rich layer with silicon in thesubstrate wherein the metal silicide interspersed in the metal richlayer reduces the amount of metal available for reacting with thesilicon in the substrate during the silicidation reaction.
 5. The methodof claim 4, wherein the dopant is silicon.
 6. The method of claim 4,wherein said metal comprises titanium.